1. Field of the Invention
This invention relates to a telecommunication system, and more particularly, to noise reduction in a long-distance optical telecommunications system.
2. Discussion of the Background
In a long-distance optical telecommunications system, the transmitted signal generally suffers from effects of nonlinearity and dispersion, which must be taken into consideration when it comes to optimizing the parameters of the system itself. On account of these effects, the signal received at the end of the communication line may have distortion (or variations of form), constituting a limitation on the system""s transmission capacity. In order to minimize the distortion, it is possible to use special transmission techniques, which depend on characteristics of the system in question, such as the bit transmission speed (or bit rate), the length of the connection, the spacing between the amplifiers and the number of WDM channels. To quote as examples of these techniques, there is chromatic dispersion compensation by means of dispersion compensation fibers or variable-pitch Bragg gratings, solitonic transmission without chromatic dispersion compensation and solitonic transmission with arrangements for chromatic dispersion compensation, as described, for example, in patent application WO99/08406 filed by the Applicant. The latter-named technique, in some cases, may represent a suitable solution for reducing the distortion in the system.
A further phenomenon, constantly present in optically amplified transmission systems, is represented by the progressive increase in amplifier spontaneous emission noise (ASE) generated along the line by the line optical amplifiers. Each time the signal passes through an optical amplifier, spontaneous emission noise is added to it. At the line end, the influence of ASE noise on the system""s performance will be correspondingly greater, the higher the level of this noise (in terms of optical power) in relation to the signal level, that is to say the lower the signal-to-noise ratio (SNR), defined as the ratio of the optical power associated with the signal to the optical power associated with the noise in a pre-established reference band of wavelength. In general, the minimum value needed for the signal-to-noise ratio in order to guarantee correct reception of the signal depends on the characteristics of the system under examination (bit rate, transmitted signal format, receiver characteristics).
When distortion and ASE noise are simultaneously present at the end of the link, the performances of the system change depending on the size of the two contributions. Generally speaking, the impairment of the system""s performances due to distortion and noise must not be in excess of established limits, beyond which correct signal reception is no longer guaranteed. In order to maintain the signal impairment within the established limits, constraints are generally imposed when defining the system parameters, and particularly when defining the bit rate, the number of WDM channels to be transmitted, the overall length of the link, the number of amplifiers to be inserted in the link and the output power of the amplifiers.
If the nonlinear effects present in the system are negligible, it may be assumed that, during propagation of the signal, there is no interaction between signal and noise and, therefore, that the ASE noise may be considered as an additional contribution to the signal. In this case, the impairment of the signal received corresponds to the combination of the impairment due to the distortion (calculated as if the ASE noise did not exist) and the impairment due to the ASE noise (calculated as if the distortion did not exist).
If, on the other hand, the nonlinear effects present in the system are not negligible, for example in the case of long-distance transmissions and/or transmissions at a high bit rate, the optical signal and the optical noise propagated along the line interact with one another. This interaction occurs due to the effect of a phenomenon known as xe2x80x9cmodulation instabilityxe2x80x9d, described for example in G. P. Agrawal, Nonlinear Fiber Optics, Academic Press, pages 134-141 and 267-273. In particular, there is modulation instability in a transmission medium if, together with the chromatic dispersion, there is a particular type of nonlinearity, known as xe2x80x9cKerr effectxe2x80x9d, which is found with the refractive index of the medium depending on the intensity of the optical signal passing through the medium itself. In the remainder of this description, when we speak of nonlinearity, we will be referring to the nonlinearities known as xe2x80x9cKerr effectxe2x80x9d.
In the case in hand, the phenomenon of modulation instability manifests itself as follows. Consider a transmission line in which there is propagation both of an optical signal S and an optical noise n. The optical noise n is a complex quantity and may be divided into a component nF in phase with the signal S and a component nQ in quadrature with the signal S. The modulation instability originating at the end of the transmission line may have different effects depending on whether the chromatic dispersion along the line is of normal or anomalous type. In the case of a line operating with anomalous dispersion, the modulation instability causes an amplification of both the in-phase noise component nE the quadrature noise component nQ to the detriment of the signal S. On the other hand, where the dispersion is of normal type, only the quadrature component nQ is amplified to the detriment of the signal S, whereas the in-phase component nF is attenuated. These phenomena are described in detail in M. Midrio, xe2x80x9cStatistical Properties of Noise Propagation in Normal Dispersion Nonlinear Fibersxe2x80x9d, J. Opt. Soc. Am. B. vol. 14, n. 11 November 1997, pages 2910-2914.
In a telecommunications system, at the end of the transmission line the signal and the noise are generally received by a quadratic type photodetector (a photodiode), in which beating occurs between the signal and the noise. In actual fact, however, the beating is only between the signal and the in-phase component nF, whereas the quadrature component nQ does not cause beating with the signal, but only with itself. This is because the electronic signal received by the photodetector is proportional to the power of the optical radiation received, that is to say to the quantity:
|S+nF+i nQ|2=S2+nF2+2xc2x7S nF+nQ2 
In this expression, S2 represents the effective signal detected by the photodiode. The other terms represent noise contributions. Usually the terms nF2 and nQ2 are negligible and, therefore, the main contribution to the noise at the receiver is given by 2xc2x7Sxc2x7nF, i.e. the term that represents the beating of the signal with the in-phase noise.
Therefore the main contribution to signal impairment due to noise comes from the beating [signal]-[in-phase noise], whereas the beatings [quadrature noise]-[quadrature noise] and [in-phase noise]-[in-phase noise] are non-influential, apart from effects of a secondary order. The presence of this type of signal impairment defines a technical problem that the Applicant has perceived as being very important in the development of optical telecommunications systems, particularly over long-distances (indicatively, distances of more than 500 km) and with high performance, for example with a bit rate greater than or equal to 2.5 Gbit/s.
With regard to continuous transmission of optical signals (i.e., on a single wavelength and with no added information), the effect of modulation instability on the noise is studied, for example, in the above article by M. Midrio. The study presented in this article confirms that, in continuous transmission of signals in a normal dispersion optical fiber, the modulation instability acts by causing a decrease of the noise in-phase component. This behavior is the opposite of that observed in an anomalous dispersion fiber, where the noise in-phase component is amplified.
The article written by R. Hui and M. O""Sullivan xe2x80x9cNoise Squeezing Due to Kerr Effect Nonlinearities in Optical Fibres with Negative Dispersionxe2x80x9d, Electronics Letters, Oct. 10, 1996, vol. 32, no. 21, pages 2001-2003, describes an experiment in which two erbium doped fiber amplifiers (EDFA) are used to amplify the continuous wave (CW) optical radiation emitted by a laser diode and to generate a given level of ASE. A wide band photodiode and a microwave spectrum analyser are used as the receiver to measure the relative intensity noise (RIN) spectrum. A positive (i.e., anomalous) dispersion optical fiber or, alternatively, a negative (i.e., normal) dispersion optical fiber is connected between the output of the second optical amplifier and the photodiode. The article demonstrates that it is possible to reduce the amplitude of the relative intensity noise RIN in systems with negative (normal) dispersion fibers. The article suggests that the physical reason for the reduction of RIN is linked to the partial coherence between the signal and the ASE due to Four Wave Mixing (FWM) in negative (normal) dispersion fibers. The article indicates that there could be practical applications for the noise squeezing in a system with negative dispersion fibers, with possible improvements in the performance of an Intensity-Modulated Direct Detection system (IM-DD).
However, the Applicant observes that, with regard to the practical arrangements for using the effect indicated in an optical telecommunications system, the article states only that the Four Wave Mixing could have an important role and that it should be taken into account when designing an appropriate dispersion compensation.
A further article by R. Hui et al., xe2x80x9cModulation Instability and Its Impact in Multispan Optical Amplified IMDD Systems: Theory and Experimentsxe2x80x9d, Journal of Lightwave Technology, Vol. 15, No. 7, July 1997, pages 1071-1081, presents a theoretical and experimental study of the effects of the nonlinear interaction between the ASE noise and the signal transmitted in a dispersive optical fiber. The article shows that, in a normal dispersion system, the nonlinearity reduces the negative effects of the ASE noise with respect to the case of linear propagation and, on the other hand, under anomalous dispersion conditions, nonlinearity always impairs system performance as compared to the case of linear propagation. The article indicates that compensation of the chromatic dispersion is an effective way of reducing the effects of modulation instability and discusses the optimal placing of the dispersion compensator. In particular, it is shown that, in a line of this type, the effects of modulation instability are reduced more with a concentrated type dispersion compensation located before the receiver than when the compensation is distributed all along the line. Concentrated compensation is produced using an optical fiber with suitable dispersion characteristics. The power of the signal input to this fiber is selected so that the production of nonlinear effects may be avoided inside the fiber.
The Applicant observes that, in this article as well, the experiments concern only continuous signal transmission.
The Applicant has noted that up to now the studies of the modulation instability phenomenon have chiefly concerned the continuous transmission of signals. The Applicant considers that these studies are not exhaustive, in the sense that they do not describe the most interesting situation in the art of optical transmissions, i.e., that in which the signal carries coded information. The Applicant has observed that, in the case of transmission of signals carrying coded information, there are signal distortion phenomena that affect the quality of the signal received and that cannot be neglected.
The article xe2x80x9cThe Effect of Dispersion Compensation on System Performance When Nonlinearities are Importantxe2x80x9d, by A. N. Pilipetskii et al., IEEE Photonics Technology Letters, Vol. 11, No. 2, February 1999, pages 284-286, asserts that the placing of dispersion compensation at the end of a nonlinear system affects both signal variance (i.e., the noise) and pulse distortion. By dint of experiments and theoretical considerations, the article demonstrates that selecting dispersion compensation to minimize the pulse distortion, rather than the signal variance, enables optimal performance to be achieved for an experimental configuration of 4780 km. The transmission fibers used in the experimental configuration have normal dispersion (xe2x88x922 ps/nm/km). The dispersion compensation is obtained in the experiment and the simulations using anomalous dispersion fibers (+17 ps/nm/km). The article concludes that, ideally, it could be possible to create a dispersion map in order to obtain noise squeezing and, at the same time, minimize pulse distortion through suitable selection of the dispersion map and through pre- and post-compensation of dispersion. In particular, the post-compensation of dispersion could be used to minimize the variance (i.e., the noise), whereas an optimization of the pulses for the corrected dispersion map could be obtained, at the same time, through correct pre-compensation of dispersion. The Applicant notes that the power values per channel indicated in the article are relatively low, that is to say insufficient to cause significant nonlinear effects in the dispersion compensation fiber DPSC added to the receiver. The Applicant also notes that the reduction of the noise effects supposed in the article is achieved using the dispersion of the optical fibers that constitute the telecommunications line and, when required, pre- and post-type dispersion compensations, together with the nonlinearity of only the optical fibers constituting the telecommunications line.
The Applicant has noted that the solutions proposed up to now to reduce the negative effects of modulation instability envisage an ad hoc selection of the dispersion compensators. These solutions require that the modulation instability problem be taken into account as early as the design stage of the transmission system, making it difficult or impossible to make changes to already installed optical systems, to increase the bit rate for example, in order for the effects of modulation instability to be taken into consideration.
The Applicant has examined the problem of supplying a technique for reducing noise that is easily and rapidly applicable to any optical telecommunications system having non-negligible optical noise, for example ASE noise, such as a long-distance system (for example, over 500 km) and/or a system with a high bit rate (greater than or equal to 2.5 Gbit/s).
Typically in an optical telecommunications system of this type, the dispersion compensation is made by alternating, along the telecommunications line, spans of transmission fiber having dispersion of opposite sign, or by inserting, usually at the optical amplifiers, suitable compensators having dispersion of opposite sign to that of the fibers constituting the telecommunications line.
The techniques and the chromatic dispersion compensation devices generally employed in optical transmission systems permit the compensation of a predetermined percentage, called ratio of compensation (RC), of the dispersion previously accumulated by the signal. In the case of transmission lines with fibers of nonuniform length and/or dispersion characteristics, the ratio of compensation (RC) is defined as the average of the ratios of compensation of the different spans of optical fiber between successive amplifiers along the line, weighted in relation to the lengths of the spans themselves. The ratio of compensation is preferably, though not necessarily, less than 100% in anomalous dispersion systems and greater than 100% in normal dispersion systems. The optimal level of the ratio of compensation depends on numerous system parameters, such as the number and length of the spans of fiber used, the coefficient of dispersion of the spans of fiber, the amount of signal pre-chirping at the transmission station, where applicable, and the optical power level of the signals transmitted.
The known art suggests that the effects of noise may be limited by exploiting the modulation instability normally present in the optical fibers used for long distance transmissions and by taking the modulation instability into consideration when designing the xe2x80x9ccompensation mapxe2x80x9d, i.e., the location and characteristics of the dispersion compensators along the line.
The Applicant has found that slight improvements may be had in this way in the system transmission capacities, but that these improvements are not significant with respect to xe2x80x9clinearxe2x80x9d transmission conditions.
As stated previously, in a normal dispersion optical fiber, modulation instability acts by causing a decrease of the noise in-phase component and a corresponding increase of the quadrature component. The Applicant has observed, however, that an anomalous dispersion optical component (constituted, for example, by an optical fiber or a chirped grating dispersion compensator) arranged along a telecommunications line comprising optical amplifiers results in a redistribution of the noise, between the two quadrature nQ and in-phase nF components, producing a substantial rebalancing of power of the components. The Applicant has found, therefore, that if an anomalous dispersion component is arranged downstream of a normal dispersion fiber, at least part of the noise transferred from the in-phase component nF to the quadrature component nQ in the normal dispersion fiber due to modulation instability is transferred in the opposite direction in the anomalous dispersion component, and there is accordingly a reduction in the effectiveness of the noise squeezing obtained previously. In a telecommunications line where the dispersion is compensated by means of alternating normal dispersion optical fibers and anomalous dispersion components (such as optical fibers or dispersion compensators with variable pitch grating), the abovementioned phenomenon of noise transfer in one direction and then in the other between the two in-phase and quadrature components is repeated numerous times and the effect of the noise squeezing at line end is relatively reduced.
The Applicant has determined that it is possible to improve system performance considerably, beyond the values corresponding to linear transmission conditions, and accordingly to overcompensate the effects of noise, by using nonlinearity combined with normal dispersion, concentrated at the end of the transmission line. This technique may be used in addition to the usual dispersion compensation techniques indicated above.
The Applicant has observed in particular that the reduction in optical noise obtainable by subjecting the optical signal at the end of the transmission line to suitably selected nonlinear phase shifting and normal dispersion may produce greater benefit than the negative effects due to the increase in distortion accordingly introduced.
The Applicant has determined that the dispersion compensation may be advantageously selected, on the basis of the previously mentioned parameters (number and length of the spans of fiber used, coefficient of dispersion of the spans of fiber, amount of the signal pre-chirping, where applicable, and optical power level of the transmitted signals), so as to obtain a sufficiently low distortion, without taking the effects of modulation instability on the noise into account. An acceptable distortion is that corresponding to a closing of the eye diagram of less than 2 dB. Preferably, however, the dispersion of the communication system is selected in such a way as to guarantee a closing of the eye diagram less than or equal to 1 dB.
In general, the Applicant has found that the distortion of signals in dispersion compensation systems may be effectively reduced by using a RC value of between 80% and 120%.
Preferably the ratio of compensation is between 85% and 115%. More preferably, RC is between 90% and 110%.
A nonlinear filter according to the invention comprises a normal dispersion and nonlinear component suitable for reducing the noise component in phase with the signal, suitable for being connected at the end of a dispersion-compensated optical telecommunications line. The nonlinear filter of the invention may also comprise an anomalous dispersion component disposed upstream of the normal dispersion and nonlinear component, suitable for correcting the shape of the pulses making up the signal.
The device of the invention is applied preferably to telecommunications systems suitable for transmitting RZ format digital signals, i.e., signals formed by pulses of lesser duration than the period corresponding to the data rate adopted, modulated on the basis of the digital information to be transmitted. In the remainder of this description, reference will be made in particular to solitonic or almost solitonic-type RZ signals, namely signals in which the pulse time shape is of the sech2(t) or similar type (for example, Gaussian), which are of special interest in the development of new long-distance transmission systems.
The Applicant has furthermore developed a method for reducing noise, comprising the step of feeding a signal transmitted on an optical telecommunications line end substantially free of distortion, before its reception, to a device having both characteristics of normal dispersion and characteristics of nonlinearity. This method may be used for reducing the optical noise in telecommunications systems substantially compensated in dispersion, and is advantageously applicable to telecommunications systems already designed or installed, permitting to optimize the performance of the transmission system in relation to noise in a way that is independent of the optimization of its other technical characteristics, such as for example dispersion.
The method of the invention is particularly suitable for systems operating with RZ type digital signals.
The method and device of the invention allow the signal-to-noise ratio at the end of the transmission line to be increased and, accordingly, the quality of the signal received to be improved, without having to alter the system parameters. This leads to the possibility of obtaining, for a like quality of the signal received, a transmission system having a greater overall length and/or a greater distance between the amplifiers and/or a higher bit rate per channel and/or a greater number of WDM channels transmitted.
According to one aspect of the invention, a method is provided for reducing noise in a long distance optical telecommunications system. The method comprises transmitting an optical signal on an optical fiber telecommunications line that comprises optical amplifiers and has a substantially compensated dispersion. The method also includes receiving from the line a noisy optical signal that includes the transmitted optical signal and an optical noise signal. The method encompasses generating an electronic signal correlated with the noisy optical signal. The electronic signal is associated with a quality parameter (Q) that depends on the optical noise signal and distortion of the optical signal. Additionally, the method includes applying to the optical signal a nonlinear phase-shift ei"psgr" associated with a variation of phase "psgr" greater than 0.5 radiants, and a normal dispersion xcex22,NORMxc2x7L, the nonlinear phase-shift and the normal dispersion being operatively selected to increase the quality parameter by at least 1 dB.
According to one embodiment of the present invention, the normal dispersion is less than 500 ps2, less than 200 ps2, or less than 100 ps2.
According to another embodiment of the present invention, the dispersion of the telecommunications line is compensated according to a compensation ratio of between 80% and 120%, between 85% and 115%, or between 90% and 110%.
According to another embodiment of the present invention, the step of transmitting the optical signal comprises the step of transmitting an optical signal carrying coded information, preferably an RZ type optical digital signal.
According to another embodiment of the present invention, the step of applying comprises applying the nonlinear phase-shift, then the normal dispersion.
According to another embodiment of the present invention, the method further comprises applying an anomalous dispersion to the optical signal and the optical noise signal.
According to another embodiment of the present invention, the anomalous dispersion may have a value between xe2x88x921000 ps2 and 0 ps2, or between xe2x88x92500 ps2 and 0 ps2.
According to another embodiment of the present invention, the method comprises amplifying the optical signal to a pre-established power level.
According to another embodiment of the present invention, the method comprises wavelength filtering the optical signal.
According to another embodiment of the present invention, the method comprises transmitting a plurality of optical signals at different wavelengths and receiving the plurality of optical signals.
According to another embodiment of the present invention, the method comprises separating the optical signals along distinct optical paths, and applying the nonlinear phase-shift and the normal dispersion along one of the optical paths.
According to another embodiment of the present invention, the step of applying is along each of the optical paths.
According to another aspect of the invention, an optical telecommunications system comprises a transmitter that is configured to generate an optical signal. An optical link comprises an optical amplifier that is configured to transmit the optical signal in a pre-established direction of propagation with substantially compensated dispersion to have an associated optical noise. A receiver is configured to receive a noisy optical signal including the transmitted optical signal and an optical noise signal. The receiver comprises a photodetecting device that is configured to generate an electronic signal correlated with the noisy optical signal. The electronic signal is associated with a quality parameter (Q) that depends on the optical noise signal and distortion of the optical signal in the optical link. The optical link further comprises a dispersive and nonlinear filtering device that includes a normal dispersion and nonlinear component. The filtering device is placed along the optical link and has an associated normal dispersion parameter xcex22,NORMxc2x7L and a nonlinearity parameter xcex3 operatively selected so as to increase the quality parameter by at least 1 dB.
According to another embodiment of the present invention, the optical signal is an RZ type digital signal.
According to another embodiment of the present invention, the normal dispersion and nonlinear component comprises a first nonlinear element and a second normal dispersion element, wherein the first element is disposed upstream of the second element along the direction of propagation.
According to another embodiment of the present invention, the filtering device comprises an anomalous dispersion component that is connected in a cascade fashion with the normal dispersion and nonlinear component and disposed upstream of the normal dispersion and nonlinear component along the direction of propagation.
According to another embodiment of the present invention, the filtering device comprises a first optical amplifier that is configured to amplify the optical signal to a pre-established power level, the first optical amplifier being disposed upstream of the normal dispersion and nonlinear component along the direction of propagation.
According to another embodiment of the present invention, the filtering device comprises a band-pass optical filter.
According to another embodiment of the present invention, the optical link comprises another optical amplifier that is disposed upstream of the filtering device along the direction of propagation and suitable for amplifying the optical signal.
According to another embodiment of the present invention, the system comprises a plurality of transmitters configured to transmit a plurality of optical signals at different wavelengths, and a plurality of receivers configured to receive the optical signals.
According to another embodiment of the present invention, the system comprises an optical signal multiplexing device that is disposed upstream of the optical transmission line along the direction of propagation, and an optical signal demultiplexing device that is disposed downstream of the optical transmission line along the direction of propagation, wherein the filtering device is disposed downstream of the demultiplexing device along the direction of propagation.
According to another embodiment of the present invention, the optical link comprises a chromatic dispersion compensator.
According to another embodiment of the present invention, the normal dispersion and nonlinear component is formed by an optical fiber.
According to another embodiment of the present invention, the first nonlinear element is an optical fiber.
According to another embodiment of the present invention, the first nonlinear element is a semiconductor device.
According to another embodiment of the present invention, the second normal dispersion element is an optical fiber.
According to another embodiment of the present invention, the second normal dispersion element comprises a Bragg grating.
According to another embodiment of the present invention, the anomalous dispersion component is an optical fiber.
According to another embodiment of the present invention, the anomalous dispersion component comprises a Bragg grating.
In yet another aspect of the invention, a device for providing the reduction of noise in a compensated dispersion optical telecommunications system comprises a receiver that is configured to receive a noisy optical signal from a compensated dispersion optical link. The noisy optical signal includes an optical signal and an optical noise signal. The receiver comprises a photodetector that is configured to generate an electronic signal correlated with the noisy optical signal. The electronic signal is associated with a quality parameter (Q) that depends on a level of distortion associated with the noisy optical signal in the optical link. An optical input is configured to optically connect to the optical link. An optical output is configured to being optically connected to the receiver. The optical output comprises a nonlinear component with normal dispersion characteristics that has a normal dispersion parameter xcex22,NORMxc2x7L and a nonlinearity parameter xcex3 operatively selected so as to increase the quality parameter by at least 1 dB.